Birefringent or polarization interference filters were first used in solar research where sub-angstrom spectral resolution is required to observe solar prominences. The first type of birefringent filter was invented by Lyot (Lyot, B. (1933) Comptes rendus 197:1593) in 1933. The basic Lyot filter (Yariv, A. and Yeh, P. (1984) Optical Waves in Crystals, Chapter 5, John Wiley and Sons, New York) can be decomposed into a series of individual filter stages. Each stage consists of a birefringent element placed between parallel polarizers. The exit polarizer for a particular stage acts as the input (or entrance) polarizer for the following stage. In a Lyot-type filter, fixed birefringent elements are oriented with optic axes parallel to the interface and oriented at 45.degree. to the direction of the input polarization. The thickness, and therefore the retardation of the birefringent elements, increases geometrically in powers of two for each successive stage in the conventional Lyot geometry Lyot-like filters in which thicknesses of the birefringent elements in successive stages increase in various arithmetic, geometric and other mathematic progressions, such as in a Fibinacci series (i.e., 1,1,2,3,5 . . . ) have been implemented. Multiple stage devices have been demonstrated with high resolution (0.1 angstrom) and broad free-spectral-range (FSR) (entire visible spectrum) (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815).
More recently, research in optical filters has focused on tuning the wavelength of peak transmission. An optical filter which can be rapidly tuned has applications in remote sensing, signal processing, displays and wavelength division multiplexing. Tunability of otherwise fixed frequency Lyot filters has been suggested and implemented using various techniques (Billings, B. H. (1948) J. Opt. Soc. Am. 37:738; Evans, J. W. (1948) J. Opt. Soc. Am. 39:229; Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815). These include mechanical methods such as stretching plastic sheets in series with the birefringent elements (Billings, B. H. (1948) J. Opt. Soc. Am. 37:738), mechanically rotating waveplates or sliding wedge plates (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. and Evans, J. W. (1948) J. Opt. Soc. Am. 39:229), changing the retardation of the birefringent elements by temperature tuning the birefringence, or changing the birefringence using electro-optic modulators (Billings, B. H. (1948) J. Opt. Soc. Am. 37:738). Temperature tuning and mechanical tuning methods are inherently slow. Electro-optic tuning of known filter devices, while much more rapid, requires large drive voltages and is limited in bandwidth by material breakdown voltages for the thin birefringent elements required (Weis, R. S. and Gaylord, T. K. (1987) J. Opt. Soc. Am. 4:1720).
Other electronically tunable filters, which have been demonstrated include acousto-optic tunable filters (AOTF) (Harris, S. E. and Wallace, R. W. (1969) J. Opt. Soc. Am. 59:744; Chang, I. C. (1981) Opt. Eng. 20:824), electro-optic tunable filters (EOTF) (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391; Lotspeich, J. F. et al. (1981) Opt. Eng. 20:830), multiple-cavity Fabry-Perot devices (Gunning, W. (1982) Appl. Opt. 21:3129) and hybrid filters such as the Fabry-Perot electro-optic Solc filter (Weis, R. S. and Gaylord, T. K. (1987) J. Opt. Soc. Am. 4:1720).
The operation of the AOTF is based on the interaction of light with a sound wave in a photoelastic medium. Strong acousto-optic interaction only occurs when the Bragg condition is satisfied. Therefore, only one spectral component of incident radiation is diffracted from the structure at a given acoustic frequency. Tuning is accomplished by changing the acoustic frequency. This was the first electrically tunable filter, which succeeded in varying the transmission wavelength from 400 nm to 700 nm by changing the acoustic frequency from 428 MHz to 990 MHz with a bandwidth of approximately 80 nm (Harris, S. E. and Wallace, R. W. (1969) J. Opt. Soc. Am. 59:744). Current AOTF's have 12.degree. fields of view, high throughput, high resolution and broad tunability (Chang, I. C. (1981) Opt. Eng. 20:824). However, power requirements are high for many applications (on the order of 10 watts/cm.sup.2) and frequency shifts induced by the filter prohibit the use of AOTF's in laser cavities. Furthermore, in AOTF's there is a tradeoff between resolution and tuning speed.
The electro-optic tunable filter (EOTF) consists of a Y-cut LiTaO.sub.3 platelet, placed between crossed polarizers, with an array of separately addressable finger electrodes (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391). Tunability is accomplished by applying a spatially periodic (sinusoidal) voltage to the 100 electrodes. Current applications of this device, however, utilize more elaborate programmable passband synthesis techniques (Lotspeich, J. F. et al. (1981) Opt. Eng. 20:830). While the power requirements for the EOTF are low, it suffers from a small clear aperture and field-of-view. This is also the main disadvantage of the Fabry-Perot devices.
Color switching has been described in liquid crystal displays which incorporate dichroic dyes (see: e.g. Aftergut et al. U.S. Pat. No. 4,581,608). Buzak U.S. Pat. No. 4,674,841 refers to a color filter switchable between three output colors incorporating a variable retarder which is a twisted nematic liquid crystal cell. Nematic liquid crystals have also been used for tuning optical filters (Kaye, W. I., U.S. Pat. No. 4,394,069; Tarry, H. A. (1975) Elect. Lett. 18:47; Gunning, W. (1980) Proc. SPIE 268:190; and Wu, S. (1989) Appl. Opt. 28:48). A disadvantage of these filters is their slow tuning speed (.about.100 ms), which is a particular problem in high resolution applications since the switching speed decreases as the order of the retarder increases. These filters are also limited in spatial resolution and angular viewing capability.
The surface stabilized ferroelectric liquid crystal (SSFLC) cell has been shown to possess properties useful in optical shutters and a number of other opto-electronic device applications requiring high contrast ratio or large modulation depth. These include electro-optic shutters, spatial light modulators for opto-electronic computing, and flat panel display devices. In such devices, the speed of response is often important. This response speed is given approximately by the equation: ##EQU1## where .tau. is the optical response (10%-90%) to an applied electric field of magnitude E, .eta. is the orientational viscosity, and P is the ferroelectric polarization density. FLC cells combine moderately fast switching speeds, with low voltage requirements and high contrast.
The physics and operation of the surface stabilized FLC (SSFLC) cells have been extensively described (Clark, N. A. et al. (1983) Mol. Cryst. Liq. Cryst. 94:213; and in U.S. Pat. Nos. 4,367,924, 4,563,059, 4,813,767, 4,840,463 and 4,958,916 all of Clark and Lagerwall). An SSFLC cell is typically formed of uniformly-spaced transparent or semi-transparent retaining walls of an inert substrate, like glass. The inside surface of the substrate walls is provided with transparent or semi-transparent electrodes. A FLC composition, often a mixture of components, is inserted between the uniformly-space transparent electrodes and the FLC molecules are aligned with respect to the substrate walls and electrodes. In an SSFLC, smectic layers are aligned perpendicular to the substrate walls which bound the FLC layer. In a SmC* SSFLC the molecular director n, i.e., the optic axis of the cell, makes an angle .alpha. to the smectic layer normal ( z) in the plane of the substrate walls. Application of an appropriate electric field to the cell electrodes allows selection between two n orientation states separated by 2.alpha.. For many SmC* FLC mixtures, .alpha.=.+-.22.5.degree., and such SmC* SSFLC cells can act like retarders which can be electronically rotated by 45.degree.. The voltage requirements for SSFLC switching devices are modest (.+-.10 V), and power consumption is quite low because the voltage need not be applied to maintain the FLC in the switched state, i.e., the devices are bistable (Clark, N. A. and Lagerwall, S. T. (1980) Appl. Phys. Lett. 36:899). Typical switching times for SmC* SSFLC cells are &lt;44 .mu.s at room temperature (ZLI-3654 mixture available from E. Merck, D-6100 Darmstadt 1, Frankfurter, Strabe, 250, F.R.G.).
Light valves have also been based upon the so called electroclinic effect in chiral smectic A (SmA*) LC materials. These devices exhibit several attractive features (see, Anderson et al. (1987) Appl. Phys. Lett. 51:640), including very fast response and voltage regulated gray scale. A number of SmA* materials have been shown to display an electroclinic effect when incorporated into SSFLC type cells. The applied voltage induces a rotation of the optic axis in these materials in an analog fashion up to a maximum .alpha..sub.MAX. The effect is described as being linear in applied voltage with very rapid response. SmA* materials having .alpha..sub.MAX up to about 22.5.degree. are known in the art, although .alpha..sub.MAX for most SmA* materials is considerably less than 22.5.degree..
The distorted helix ferroelectric (DHF) effect has been described with smectic C* liquid crystals having a short pitch (see: Ostrovski and Chigrinov (1980) Krystallografiya 25:560; Ostrovski et al. in Advances in Liquid Crystal research and Application, (L. Bata, ed.) Pergamon, Oxford; Funfschilling and Schadt (1989) J. Appl. Phys. 66:3877). In SSFLC cells incorporating certain short-pitch materials, the helix of the material is not suppressed, and thus the helix can be distorted by the application of an electric field. This distortion results in a electric field-dependent, spatially-averaged change in the tilt angle of the material coupled with a voltage-dependent variation in spatially averaged birefringence. DHF cells are attractive since induced tilt angles as high as about .+-.38.degree. can be attained with applied voltages lower than those required for SmA* electroclinic and SmC* SSFLC cells. Beresnev et al., EPO Patent Application EP 309,774, published Apr. 5, 1989, describe DHF cells. DHF materials are further described in Buchecker et al., EP 339,414, published Nov. 2, 1989.
Ozaki et al. (1985) Jpn. J. Appl. Phys. (part 24 (suppl.24-3):63 refers to a high speed color switching element in which dichroic dyes are mixed with ferroelectric liquid crystals. Color switches and or displays which combine color filters and ferroelectric liquid crystal cell shutters have been described. See: e.g. Seikimura et al. U.S. Pat. No. 4,712,874; Takao et al. U.S. Pat. No. 4,802,743; Yamazaki et al. U.S. Pat. No. 4,799,776; Yokono et al. U.S. Pat. No. 4,773,737.
Carrington et al. (1989) Second International Conference on Ferroelectric Liquid Crystals Program and Abstracts (Goteborg, Sweden, 27-30 June 1989) Abstract 015 refers to rapid switching of spatial arrays of FLC two color switches in color displays.
Lagerwall et al. (1989) "Ferroelectric Liquid Crystals: The Development of Devices" Ferroelectrics 94:3-62 is a recent review of the use of FLC cells in device applications. In a section called "SSFLC Color" the reviewers refer to color display (e.g. for television applications). Matsumoto et al. (1988) SID88 Digest, 41, refers to color generation via pixel subdivision using FLC cells. Each pixel of a display is divided into three (or more) sub-pixels of blue, green and red. Disadvantages of this technique for color generation include a reduction in resolution and the complexity of fabrication of large, high resolution displays. Ross (1988) International Display Research Conference (1988) 185 refers to color sequential backlighting using FLC cells. This method is implemented by switching between blue, green and red images at sufficient rates that the eye averages the primary color images. The method involves switching of a wavelength selective source synchronously with images on a liquid crystal display. Three primary colors (usually red, green and blue) define an area in color space. Desired colors in the area can be displayed by controlling the level of primary colors in each pixel. Backlighting liquid crystal displays uses fluorescent tubes with fast phosphors (White (1988) Phys. Technol. 19:91).
Johnson and Sharp in U.S. patent application Ser. No. 07/522,215 and PCT application WO91/06888 published May 16, 1991, describe birefringent or polarization interference filters employing chiral smectic liquid crystal cells as electronically rotatable retarders. Discretely tunable and continuously tunable single and multiple stage filters are described.